Currently, the amount of information to be transmitted through optical communications keeps increasing. In order to match up to such an increase in amount of information, some measures are being taken. Such measures includes, for example, increasing a signaling rate and increasing the number of channels by use of wavelength-division multiplexing. Among such measures, there is a next-generation 100-Gbps digital coherent transmission technology. This technology employs polarization multiplexing in which different pieces of information are superimposed on two polarized waves whose electric fields are orthogonal to each other, so as to double the amount of information transmittable per unit of time. However, a modulation method employing such polarization multiplexing requires an optical modulator having a complex configuration. This results in problems such as increases in device size and device cost. In view of these problems, Non-Patent Literature 1 discloses an optical modulator employing a substrate-type optical waveguide with use of silicon as a core. Such a substrate-type optical waveguide has, for example, the following advantages: a production process is simple; the size of an optical element can be reduced by integration; and production costs can be reduced by use of a large-diameter wafer.
In the polarization multiplexing, a polarization beam combiner (PBC) is used. The PBC carries out multiplexing of a TE polarized wave and a TM polarized wave within the substrate-type optical waveguide. (a) of FIG. 36 is a block diagram illustrating a configuration of a PBC 101. The PBC 101 includes a first input port 102, a second input port 103, and an output port 104. The PBC 101 carries out multiplexing of a TM polarized wave introduced through the first input port 102 and a TE polarized wave introduced through the second input port 103, and outputs, through the output port 104, the TM polarized wave and the TE polarized wave which are thus multiplexed. Note that the length of each arrow illustrated in (a) of FIG. 36 indicates the power of each of the TE polarized wave and the TM polarized wave which enter into the PBC 101. In (b) and (c) of FIG. 36 which are referred to later, the length of each arrow similarly indicates the power of each polarized wave.
The TE polarized wave herein refers to a mode having an electric field whose main component is present along a horizontal direction (hereinafter, referred to as a width direction or x direction) parallel to a substrate in a plane orthogonal to a light traveling direction in the substrate-type optical waveguide. Meanwhile, the TM polarized wave herein refers to a mode having an electric field whose main component is present along a direction (hereinafter, referred to as a height direction or y direction) perpendicular to the substrate in the plane orthogonal to the light traveling direction in the substrate-type optical waveguide.
In terms of performance of PBCs, loss during polarization multiplexing and a polarization extinction ratio are important.
The loss of a TM polarized wave indicates a ratio of the power of the TM polarized wave at the output port 104 with respect to the power of the TM polarized wave introduced through the first input port 102. The loss of a TM polarized wave is defined by the following Formula (1).
                    [                  Math          .                                          ⁢          1                ]                                                            LOSS        =                              -            10                    ⁢                                          ⁢                      Log            10                    ⁢                                                                                          POWER                    ⁢                                                                                  ⁢                    OF                    ⁢                                                                                  ⁢                    TM                    ⁢                                                                                  ⁢                    POLARIZED                                                                                                                    WAVE                    ⁢                                                                                  ⁢                    AT                    ⁢                                                                                  ⁢                    OUTPUT                    ⁢                                                                                  ⁢                    PORT                                                                                                                                            POWER                    ⁢                                                                                  ⁢                    OF                    ⁢                                                                                  ⁢                    TM                    ⁢                                                                                  ⁢                    POLARIZED                                                                                                                    WAVE                    ⁢                                                                                  ⁢                    AT                    ⁢                                                                                  ⁢                    INPUT                    ⁢                                                                                  ⁢                    PORT                                                                                                          (        1        )            
Meanwhile, the loss of a TE polarized wave indicates a ratio of the power of the TE polarized wave at the output port 104 with respect to the power of the TE polarized wave introduced through the second input port 103. The loss of a TE polarized wave is defined by the following Formula (2).
                    [                  Math          .                                          ⁢          2                ]                                                            LOSS        =                              -            10                    ⁢                                          ⁢                      Log            10                    ⁢                                                                                          POWER                    ⁢                                                                                  ⁢                    OF                    ⁢                                                                                  ⁢                    TE                    ⁢                                                                                  ⁢                    POLARIZED                                                                                                                    WAVE                    ⁢                                                                                  ⁢                    AT                    ⁢                                                                                  ⁢                    OUTPUT                    ⁢                                                                                  ⁢                    PORT                                                                                                                                            POWER                    ⁢                                                                                  ⁢                    OF                    ⁢                                                                                  ⁢                    TE                    ⁢                                                                                  ⁢                    POLARIZED                                                                                                                    WAVE                    ⁢                                                                                  ⁢                    AT                    ⁢                                                                                  ⁢                    INPUT                    ⁢                                                                                  ⁢                    PORT                                                                                                          (        2        )            
In view of energy efficiency, the losses are each preferably a low loss.
On the other hand, the polarization extinction ratio (hereinafter, also referred to as “PER”) indicates a ratio of the power of a TM polarized wave and the power of a TE polarized wave which are outputted through the output port 104 in a case where the TM polarized wave and the TE polarized wave are introduced through one (e.g., the second input port 103) of input ports of a PBC (see (b) of FIG. 36. The length of each arrow in (b) of FIG. 36 is indicative of the power of each polarized wave).
In a case where a TM polarized wave and a TE polarized wave of the same power are introduced through the first input port 102 which is for input of the TM polarized wave (see (b) of FIG. 36), the PER is defined by the following Formula (3).
                    [                  Math          .                                          ⁢          3                ]                                                            PER        =                  10          ⁢                                          ⁢                      Log            10                    ⁢                                                                                          POWER                    ⁢                                                                                  ⁢                    OF                    ⁢                                                                                  ⁢                    TM                    ⁢                                                                                  ⁢                    POLARIZED                                                                                                                    WAVE                    ⁢                                                                                  ⁢                    AT                    ⁢                                                                                  ⁢                    OUTPUT                    ⁢                                                                                  ⁢                    PORT                                                                                                                                            POWER                    ⁢                                                                                  ⁢                    OF                    ⁢                                                                                  ⁢                    TE                    ⁢                                                                                  ⁢                    POLARIZED                                                                                                                    WAVE                    ⁢                                                                                  ⁢                    AT                    ⁢                                                                                  ⁢                    INPUT                    ⁢                                                                                  ⁢                    PORT                                                                                                          (        3        )            
In a case where a TM polarized wave and a TE polarized wave of the same power are introduced through the first input port 102 for input of the TE polarized wave (see (c) of FIG. 36), the PER is defined by the following Formula (4).
                    [                  Math          .                                          ⁢          4                ]                                                            PER        =                  10          ⁢                      Log            10                    ⁢                                    POWER              ⁢                                                          ⁢              OF              ⁢                                                          ⁢              TE              ⁢                                                          ⁢              POLARIZED              ⁢                                                                                ⁢                                                                              ⁢              WAVE              ⁢                                                          ⁢              AT              ⁢                                                          ⁢              OUTPU              ⁢                                                          ⁢              PORT                                                                                  ⁢                                                                                          POWER                      ⁢                                                                                          ⁢                      OF                      ⁢                                                                                          ⁢                      TM                      ⁢                                                                                          ⁢                      POLARIZED                                                                                                                                  WAVE                      ⁢                                                                                          ⁢                      AT                      ⁢                                                                                          ⁢                      OUTPUT                      ⁢                                                                                          ⁢                      PORT                                                                                  ⁢                                                                                                      (        4        )            
As described above, the PER is indicative of a degree of suppression of the power of one of a TM polarized wave and a TE polarized wave in a case where the TM polarized wave and the TE polarized wave are introduced through one input port. The PER is important, for example, in the following point. A PCB, like a polarization multiplexing modulator disclosed in Non-Patent Literature 1, is connected to a subsequent stage of a polarization rotator (hereinafter, also referred to as “PR”). The PR is a device for converting a TE polarized wave into a TM polarized wave. However, the TE polarized wave is slightly mixed in the TM polarized wave that is to be outputted from the PR, due to insufficient conversion. The TE polarized wave thus mixed in the TM polarized wave causes, at the output port 104 of the PBC 101, crosstalk with a TE polarized wave (TE polarized wave illustrated in (a) of FIG. 36) that is to be multiplexed. This crosstalk results in deterioration of signal quality. Accordingly, it is preferable to make the PBC 101 suppress the occurrence of such crosstalk at the output port 104, by suppression of the power of the TE polarized wave which has been mixed in a PR output and introduced. In other words, the higher the PER is, the more the occurrence of crosstalk at the output port 104 can be suppressed. This consequently makes it possible to reduce deterioration of signal quality in polarization multiplexing.
It is preferable that the above-described two items of performance of PBCs be favorable in a wide wavelength band. This is for the following reason. In optical communications, wavelength multiplexing is widely used. Accordingly, many optical components including an optical modulator preferably operate in a wide wavelength band. The wide wavelength band means a band including, for example, C band (a wavelength range of 1530 nm to 1565 nm) and L band (a wavelength range of 1565 nm to 1625 nm). In a case where the PBC is utilized in such an optical component, it is preferable that the PBC also have a low loss and a high PER in a wide wavelength band.
Literatures on conventional technologies of PBCs include Non-Patent Literature 2 and Patent Literature 1.
Non-Patent Literature 2 relates to a polarization beam splitter. The polarization beam splitter can be obtained by causing a TE polarized wave and a TM polarized wave to enter through the output port 104 of the PBC 101 illustrated in (a) of FIG. 36 and then causing the TM polarized wave to exit through the first input port 102 and the TE polarized wave to exit through the second input port 103. As described above, the polarization beam splitter can achieve a function that is equivalent to the function of a PBC. Therefore, the polarization beam splitter is taken as a conventional art of PBCs. Non-Patent Literature 2 achieves polarization separation of TE0 and TM0 by a directional coupler in which two rectangular waveguides having congruent core shapes are provided adjacent to each other. Here, TE0 indicates a waveguide mode of a TE polarized wave which waveguide mode has a maximum effective refractive index among waveguide modes of the TE polarized wave, while TM0 indicates a waveguide mode of a TM polarized wave which waveguide mode has a maximum effective refractive index among waveguide modes of the TM polarized wave. FIG. 37 is a schematic view illustrating a configuration of a polarization beam splitter 201 disclosed in Non-Patent Literature 2. (a) of FIG. 37 is a cross-sectional view of a directional coupler of the polarization beam splitter 201, along a cross section orthogonal to a light traveling direction. (b) and (c) of FIG. 37 each are a top view of the polarization beam splitter 201. The polarization beam splitter 201 includes a lower cladding 204, an upper cladding 205, and cores 202 and 203 which are buried by the lower cladding 204 and the upper cladding 205.
The polarization beam splitter 201 allows for multiplexing or separation of polarized waves by utilizing the following phenomenon: in a directional coupler, a coupling length for TM0 is shorter than a coupling length for TE0. More specifically, the polarization beam splitter 201 allows for multiplexing ((c) of FIG. 37) or separation ((b) of FIG. 37) of polarized waves by utilizing the following phenomenon: TM0 completely transfers over to an adjacent waveguide of a directional coupler before TE0 completely transfers over to the adjacent waveguide.
Patent Literature 1 relates to a polarization sorter, and can perform an operation equivalent to that of PBCs. Thus, the polarization sorter is taken as a conventional art of PBCs. The polarization sorter disclosed in Patent Literature 1 carries out polarization separation by mode sorting (adiabatic sorting) which utilizes an adiabatic conversion.
As illustrated in FIGS. 2a to 2c of Patent Literature 1, the polarization sorter in accordance with Patent Literature 1 includes two waveguides 12 and 14. The waveguides 12 and 14 are provided adjacent to each other, and have respective cores whose heights are different from each other. Further, the polarization sorter includes a mode sorting section 46 in which one of the cores has a width changing continuously along a light traveling direction. The mode sorter carries out polarization separation by mode sorting in the mode sorting section 46. The mode sorting here means a polarization separation method which utilizes the following phenomenon: a magnitude relation of effective refractive indexes and polarization are preserved when the waveguides are arranged to continuously change along the light traveling direction.
For example, a magnitude relation of effective refractive index between a TE polarized wave (TE-1 illustrated in FIG. 6 of Patent Literature 1) at an input port 30 of a waveguide 12 and a TE polarized wave (TE-2 illustrated in FIG. 6 of Patent Literature 1) at an input port 36 of a waveguide 14 is opposite to that between a TE polarized wave at an output port 32 of waveguide 12 and a TE polarized wave at an output port 34 of waveguide 14. Meanwhile, a magnitude relation of effective refractive index between a TM polarized wave (TM-1 illustrated in FIG. 6 of Patent Literature 1) at the input port 30 of the waveguide 12 and a TM polarized wave at the input port 36 of the waveguide 14 (TM-2 illustrated in FIG. 6 of Patent Literature 1) is the same as that between a TM polarized wave at the output port 32 of the waveguide 12 and a TM polarized wave at the output port 34 of the waveguide 14.
When the above magnitude relation of effective refractive index is satisfied, the TE polarized wave having been introduced through the input port 30 of the waveguide 12 is outputted through the output port 34 of the waveguide 14 while the TM polarized wave having been introduced through the input port 30 of the waveguide 12 is outputted through the output port 32 of the waveguide 12. In this way, the polarization sorter of Patent Literature 1 carries out polarization separation of the TE polarized wave and the TM polarized wave which have been introduced through the input port 30 of the waveguide 12.
In order to satisfy the above-described magnitude relation of effective refractive index, in the polarization sorter, cross-sectional shapes of respective cores of the two adjacent waveguides 12 and 14 cannot be congruent all along an entire device length of the polarization sorter. Therefore, as illustrated in FIGS. 2a to 2c of Patent Literature 1, the waveguides 12 and 14 adjacent to each other have different heights, respectively.